In recent years, various methods have been tried out to treat diseases without major surgery so as to lighten the physiological load on the patient and to improve the progress in recuperation. Also, even if an abdominal operation is to be performed, determination in detail of the state of diseased areas before surgery and determination of the internal state without having to cut open the organ surface during surgery would give very meaningful information at the time of actual surgery. To satisfy these needs, in recent years, ultrasonography has developed and spread remarkably. The accuracy of the diagnosis using this before surgery has been extremely helpful in improving results of surgery in recent years. In particular, striking improvements have been made in the ability to diagnose diseases of the thyroid glands by the combination of the ultrasonography and diagnosis of cells obtained by centesis and suction.
However, when trying to observe the internal state by placing the probe of the ultrasonograph directly on the body surface or organ surface, it is difficult to obtain a clear image in the region within several centimeters of the surface due to the nature of the ultrasonograph. Further, the actual body and organ surfaces are not flat, but have characteristic curves and unevenness, so it is impossible to bring an inflexible probe which maintains a certain shape into close contact with the desired location. That is, if air is present between the body and probe, the rate of propagation of the ultrasonic waves drops remarkably and an accurate image cannot be formed on the screen of the diagnostic apparatus.
To solve the above-mentioned problems, it is effective to interpose a suitable spacer (i.e., coupling medium) between the probe and body. The coupling medium is preferably in the form of a sheet and is sandwiched between the probe and the body surface at the time of diagnosis or is formed into a suitable shape and is used attached to the probe directly or with a fitting. Such a coupling medium is required to have a suitable flexibility and mechanical strength and improved acoustical characteristics (e.g., a low rate of ultrasonic attenuation etc.)
For example, Japanese Unexamined Patent Publication (Kokai) No. 55-63636 discloses a specific water-containing polymer gel. However, the gel disclosed has problems such as an insufficient mechanical strength or a large attenuation of sound waves. Various efforts have been made to solve with this problem.
For example, there are known a polyvinyl alcohol based polymer gel (Japanese Unexamined Patent Publication (Kokai) No. 62-298342, a high water absorbable resin (Japanese Unexamined Patent Publication (Kokai) No. 4-53544), and various organic and inorganic polymers (Japanese Examined Patent Publication (Kokoku) No. 2-21252).
However, these various proposed polymer gels also suffer from various problems. Specifically, media using polymer gels suffer from the danger of all or part of the gel entering and remaining in the body at the time of centesis or at the time of surgery, and therefore, there are apprehensions over the toxicity of the gel itself or the residual monomers to cause a problem in safety. Further, natural polymers and polyvinyl alcohol gels, which are considered highly safe, are not necessarily satisfactory in terms of their acoustical characteristics (for example, have a high attenuation rate). To improve this acoustical characteristics, it is necessary to raise the water content in the gel. However, when the water content is increased, there is the problem that the mechanical strength will decrease. Further, a polyvinyl alcohol gel tends to bleed water when pressure is applied, and therefore, is not suited as a gel for a probe used pressed against the body or organ surface. Further, it is poor in sterilizability (that is, it completely melts and loses its original shape under heating at 121.degree. C. in an autoclave, the simplest method of sterilization). As a result, it is not yet commercialized. In view of this situation, development of a coupling medium for a probe which is safe and can be used even at the time of centesis and during surgery has been desired.